Supercritical fluid chromatography has been reported by T. H. Gouw and R. E. Jentoft in J. Chromatogr., Vol. 68, pages 303-323, 1972, but with a packed column as an essential component of the system. Packed columns have some inherent problems that limit their usefulness. Among these problems are the large pressure drop along the column which limits the length of column that is practicable and the density gradient in the fluid which the pressure drop make inevitable. Another problem is the plate height contributions due to alternate solvent flow paths that are present in packed columns.
Open tubular confined passageways, including those of capillary dimensions, are available commercially but they have not been used for supercritical fluid chromatography.
Temperature programming is used to modify solute retention in gas chromatography. In supercritical fluid chromatography liquid formation is prevented by operating at constant temperature near or above the critical temperature of the fluid and the mobile phase density is controlled by adjusting the pressure. Pressure programming has been reported for controlling the density of the fluid used in supercritical fluid chromatography by R. E. Jentoft and T. H. Gouw in J. Chromatogr. Sci., Vol. 8, pages 138-142, 1970, and by J. E. Conway, J. A. Graham and L. B. Rogers in J. Chromatogr. Sci., Vol. 16, pages 102-110, 1978. A gradual increase in pressure gradually increases the mobile phase density but not in a linear relation. Increased mobile phase density decreases solute retention.
W. Asche has stated that a number of different mobile phases may be used in supercritical fluid chromatography in Chromatographia, Vol. 11, pages 411-412, 1978.